The properties of nanoscale systems such as molecules and clusters of atoms can be very different from the properties of bulk materials. That is because, at the nanoscale, quantum effects become important. In particular, the structural, optical, and electronic properties of atomic clusters can be size dependent. This is not only interesting from a theoretical point of view, but may also lead to new technological applications. The goal of my research is to use first-principles calculations to gain a better understanding of the role of quantum effects in nanoscale systems.